The present invention relates to aggregate processing. More particularly, this invention relates to a percentage-based computerized method and means for reblending fine aggregates, such as sand, to predetermined specifications.
Production sites in operation for decades will eventually run short of easily processable aggregate material. Many plant operators are already finding that some existing sections fail to contain sand passing their gradation specifications. Consequently, the plant operators must work around the substandard deposits or, worse yet, discard otherwise saleable material that happens to be mixed with the substandard material.
With increasing consciousness of environmental issues and expanding population areas, many producers are unable to secure additional zoning permits to continue production and must optimize their existing plants. Specifications for higher strength materials or specialty products, such as those developed by a computer, require more extensive processing methods. Producers who can consistently meet those higher demands may find these products worth several times what they are supplying now.
Fine aggregate, herein referred to as minus xe2x85x9cxe2x80x3 (9.5 mm) or number 4 mesh, cannot be separated efficiently in large quantities with vibrating screens, unlike its counterpart coarse aggregate. A traditional sand gradation specification has several different sizes required (frequently six to eight) and is written in either cumulative percent retained or percent passing through a screen. A single screening machine that could handle even 100 tons per hour just to separate these different sizes would probably be very large and cost prohibitive for most producers.
In many cases, plant operators have been able to use large quantities of water or rising currents in a fine-material washer (sand screw) to float out up to about 50 mesh material. By doing so, saleable sand products may have been produced from reasonably good deposits. In some cases, though, consistency may suffer or the material may not meet specification due to changes in the deposit. A method for separating fractions was required.
As early as the 1950s, classification tanks were being used in the United States. Early models were not much more than long, water-filled tubs with several valves to discharge accumulated material located in the floor of the vessel. Some models had manually operated valves, while others were spring loaded. Many valving methods were very messy, difficult to maintain, or prone to excessive wear. A portion of excess material from given valves was diverted away from the rest of the tank""s material, thus attempting to bring the resultant product into specification.
Today, in conventional classification tanks, rods extending from a control bridge mounted over the tank support the product valves. The valves are usually grouped into units of three, called a station or cell, and also incorporate a material level sensor to detect the availability of sand. Hydraulic cylinder pistons actuate the valve rods when the sensor trips and a tank control determines which valve opens.
In a classification tank, sand slurry is pumped into the tub and over an elevating plate at one end. The plate acts as a ramp to arc the flow through the tank. In general, heavier particles fall out of suspension from this flow while lighter particles are carried farther down the vessel. Each particle has a tendency to settle in a particular area; however, they are not immune to external influences. For example, sand slurry entering the vessel at different velocities will settle in different areas.
In most sand specifications, only a limited quantity of minus 200 mesh material (silts) can be tolerated. In most processing applications, these silts must be washed off or floated out with water at a ratio of 100 gallons per minute per ton per hour of silt. When insufficient clean water exists, the silts remain in suspension and increase the specific gravity and particles inconsistently settle. If silt dilution is not met for a long period of time, these particles will migrate down the tank and mud will eventually accumulate over and around the back end valves, thus preventing any material from existing.
A similar problem occurs with material of varying specific gravities. In hydraulic classification tanks, smaller particles travel farther, but when the particles are the same sizes, the material with a higher specific gravity will fall more quickly than its lighter companions. The same can be said for irregularly shaped or flat material such as crusher tailings. In addition, as individual grains travel down the tank, they collide with other grains, support structures or valve rods. The result could be these particles bouncing farther down the tank or decreasing in velocity and falling out of suspension sooner than expected.
It is difficult to imagine the influence of up to 500 tons per hour of collisions, material densities, surface areas, specific gravities and movement rates. In actuality, several different sizes of material settle at each station. One or two sand fractions, with a much smaller mix of other sizes, predominately comprise a sample from each valve. By examining these valve samples, a general model of a tank is seen. This gradation of the material discharged from each cell is referred to as a station analysis. Though far from perfect separation, it is reasonably consistent and is the principle basis of classification tanks today.
Given the station analysis, one has only to determine the quantities discharged over time to develop an overall picture of the tank production. The first tank controls were not much more than timers, indicator lights, and relays and served as good examples of early automation counting machines.
An operator observes how long each valve discharges material and adjusts the product split at that station to attempt to bring the product into specification. Unfortunately, this system is usually trial and error. Once the operator sets the control for given settings, he still needs to sample the final product and make adjustments to the station timers. Over time, the discharge rates change and the operator needs to adjust the control again.
Many operators set up their controls to run with very tight control and everything outside of that model is discarded. Most plants operate in this way and produce products in specification, but in some cases the waste material unnecessarily outweighs the saleable product. The need for an efficient control that could adjust itself to compensate for feed or flow changes is evident.
Therefore, a primary objective of the present invention is the provision of a method and means for an efficient, self-adjusting, flexible, closed loop computerized control system for reblending sand to a given specification.
Another objective of the present invention is the provision of a reblending control system, which utilizes programmable logic controllers connected to a computer.
Another objective of the present invention is the provision of a control system for fine aggregate reblending which controls valve opening time at each station as a percentage of the total available valve opening time at all stations.
Another objective of the present invention is the provision of a control system for fine aggregate reblending which controls valve opening time by a plurality of discharge valves at a given station as a percentage of the total available valve opening time at that particular station.
Another objective of the present invention is the provision of a control method for reblending sand that efficiently minimizes excess fine particles.
Another objective of the present invention is the provision of a control system for fine aggregate reblending which is economical to manufacture and use, is reliable in use, and provides for remote control and networking.
These and other objectives will be apparent from the drawings, as well as from the description and claims which follow.
The present invention relates to a method and means for reblending sand. The means includes a classification tank which has a plurality of stations and valves associated therewith. A computer electrically connected to a programmable logic controller (PLC) controls the valves at each station on a percentage basis.
The method of reblending aggregate includes delivering aggregate of various sizes to the classification tank having a plurality of stations with primary and secondary discharge valves. The tank is then calibrated to determine raw feed analysis, discharge rates and the flow multipliers needed to develop a mathematical model of the tank. Inputting the calibration information data and the desired production specification into a computer allows the tank to be operated and controlled with appropriate adjustments in discharge rates and valve percent settings at each station. This method provides more accurate control than the existing min-max control methods.
Inputting the data into a computer provides greater flexibility, control and accuracy. The computer and the programmable logic controllers are connected in a closed loop so that feedback can be given to the computer by the PLCs and the computer can provide command signals for opening and closing the valves. Percentage-based control allows optimization within each station and throughout the whole tank.
Two algorithms for controlling the discharge valves to bring the product into specification using percentage-based control are discussed. In both algorithms a method of reblending sand in a classification tank having multiple stations and discharge valves within each station includes the steps of delivering sand to the tank; calibrating the tank to analyze the raw feed material and develop discharge rates, flow multipliers, and a mathematical model of the tank; and inputting a specification or quantity and sieve size distribution for a reblended sand product into a computer for controlling the tank. Then the tank is operated in a production mode and the discharge valves at each station of the tank are adjusted by the computer for percent open time and discharge rate. With the first algorithm described below, the computer identifies which station has the most of a particular sieve size in excess of specification and diverts material at that station by closing the appropriate discharge valve(s).
The second algorithm reacts to an excess of fine particles in the primary product by starting at the station farthest from the inlet of the tank, where natural hydraulic settling typically places the most fine particles, and reducing the percentage open time of the primary valve by fixed amounts until the primary product is brought into specification. If adjustments result in the primary valve in that station being completely closed, the control moves a station closer to the inlet and repeats the same process until the primary product is in specification. Then the secondary product is brought into specification in a similar way. This algorithm is very effective in reducing the amount of fine particles in the products. Thus, it is referred to herein as the xe2x80x9cminimize finesxe2x80x9d method.
The operator can have the computer simulate or calculate the results with both of the algorithms and choose the one that gives the best results.